1. Field of the Invention
The present invention relates to a III-V group nitride system semiconductor self-standing substrate, a method of making the same, and a III-V group nitride system semiconductor wafer.
2. Description of the Related Art
Nitride system semiconductor materials such as gallium nitride (GaN), indium gallium nitride (InGaN) and gallium aluminum nitride (GaAlN) have a sufficiently wide bandgap and are of direct transition type in inter-band transition. Therefore, they are a great deal researched to be used for short-wavelength light emitting devices. Further, they have a high saturation drift velocity of electron and can use two-dimensional carrier gases in hetero junction. Therefore, they are also expected to be used for electronic devices.
With silicon (Si) or gallium arsenide (GaAs) which is already in popular use, an epitaxial growth layer of silicon (Si) or gallium arsenide (GaAs) to compose a device is homo-epitaxially grown on Si substrate or GaAs substrate of same kind of material. In the homo-epitaxial growth on homo-substrate, the crystal growth proceeds in step flow mode on the initial stage. Therefore, it is easy to obtain a flat and epitaxially grown surface while generating little crystal defect.
In the case that a ternary or more compound crystal layer such as AlGaInP is grown on GaAs substrate, the surface morphology of epitaxial layer is likely to be roughened. However, by intentionally inclining the plane orientation of underlying substrate from a low index surface as a reference surface to a specific direction, which is generally called “off-orientation”, it becomes possible to obtain a flat epitaxially grown surface while generating little crystal defect. The direction or angle of off-orientation can have an optimum value according to the kind of material or growth conditions of an epi-layer grown thereon, and an optimum off-direction or off-angle common to all material substrates does not exist. For example, in case of GaAs substrate, it is inclined from its (001)-face as a reference surface to [110] direction or [1-10] direction, and its off-angle varies in the range of about 0 to 20 degrees.
On the other hand, it is difficult to grow a bulk crystal of nitride system semiconductor, and a GaN self-standing substrate did not exist before the epitaxial growth of nitride is researched. Therefore, nitride system semiconductor crystal has been hetero-epitaxially grown on underlying single-crystal sapphire as a hetero-substrate by using a vapor-phase growth process such as MOVPE (metal organic vapor phase epitaxy), MBE (molecular beam epitaxy) and HVPE (hydride vapor phase epitaxy). Even now, such a process is used for the manufacture of blue LED's.
However, in the hetero-epitaxial growth on hetero-substrate, a number of dislocations (defects) must be generated in grown crystal due to a lattice mismatch between the underlying substrate and the grown crystal. Therefore, if such process is applied to a device such as a laser diode sensitive to the crystal defect, the light output lowers and the lifetime of device is shortened.
In recent years, ELO (epitaxial lateral overgrowth; e.g., Appl. Phys. Lett. 71 (18) 2638 (1997)), FIELO (facet-initiated epitaxial lateral overgrowth; e.g., Jpan. J. Appl. Phys. 38, L184 (1999)) and pendeoepitaxy (e.g., MRS Internet J. Nitride Semicond. Res. 4S1, G3.38 (1999)) are reported as a growth method for reducing a defect density generated due to the lattice mismatch between sapphire and GaN. In these methods, a SiO2 patterning mask etc. is formed on GaN grown on a sapphire substrate, and then GaN is selectively grown from the mask window. Thereby, the propagation of dislocation from underlying crystal can be suppressed. Due to such a growth method, the dislocation density in GaN can be significantly reduced to a level of 107 cm−2 or so.
Further, various methods of making a self-standing GaN substrate are suggested that a thick GaN layer with reduced dislocation density is epitaxially grown on a hetero-substrate such as sapphire and then the GaN layer grown is separated from the underlying substrate. For example, Japanese patent application laid-open No. 11-251253 discloses a method of making a self-standing GaN substrate that a GaN layer is grown on a sapphire substrate by ELO and then the sapphire substrate is removed by etching.
Other than this, VAS (Void-Assisted Separation: e.g., Y. Oshima et al., Jpn. J. Appl. Phys. Vol. 42 (2003) pp. L1-L3, Japanese patent application laid-open No. 2003-178984) and DEEP (Dislocation Elimination by the Epi-growth with inverted-Pyramidal pits: e.g., K. Motoki et al., Jpn. J. Appl. Phys. Vol. 40 (2001) pp. L140-L143, Japanese patent application laid-open No. 2003-165799) are known. The VAS is conducted such that GaN is grown through TiN thin film with a mesh structure on substrate such as sapphire while providing voids at the interface of underlying substrate and GaN layer, thereby allowing both the separation and the dislocation reduction of GaN substrate. The DEEP is conducted such that GaN is grown on a GaAs substrate, which is removable by etching, by using a SiN patterning mask while intentionally forming pits surrounded by facets on the surface of crystal, accumulating dislocations at the bottom of pits to allow regions other than pits to have a low dislocation density.
However, even when such a GaN self-standing substrate is used to grow a GaN system epitaxial layer thereon, it is difficult to flatten its surface morphology in the epitaxial growth while offering high flatness, uniformity and reproducibility.
To use an off-oriented GaN self-standing substrate may be thought in flattening the surface morphology of epi-layer of nitride system semiconductor grown thereon, as in the case of the other compound semiconductor substrate such as GaAs substrate.
However, even when using a hetero-substrate properly off-oriented, a GaN self-standing substrate obtained must be subjected to a dispersion in off-orientation due to a distortion (warping) of substrate generated during the crystal growth since it is made by separating a thick crystal grown hetero-epitaxially from the hetero-substrate, different from the case of a GaAs substrate that a wafer can be cut off from an ingot grown as a bulk crystal. Therefore, any GaN substrate with an in-plane uniform off-orientation cannot be obtained yet. When III-V group nitride system semiconductor crystal is epitaxially grown on such an off-oriented GaN substrate, due to the dispersion in off-orientation, the surface morphology of epitaxial growth layer will be roughened in the plane of substrate, and the composition of mixed crystal or the impurity concentration will be dispersed. These are unique problems on nitride system semiconductor self-standing substrates.